When an electronic apparatus is operated, energy required therefor is taken into the electronic apparatus. However, it is difficult to allow all of the taken energy to be consumed for operating the electronic apparatus. A part of the energy is consumed as thermal energy or the like without achieving the original object. It is expected that the energy consumed in this way is once stored as electrical energy in a storage element, and the stored energy is reused if necessary, thereby reducing consumed energy and increasing efficiency.
In order to do so, a storage element from which energy necessary for an operation of an electronic apparatus can be taken out at an appropriate output is needed. Examples of the storage element include a capacitor and a secondary battery. Among them, in particular, much attention has been paid to electric double layer capacitors that have large capacitance, are capable of rapid charge and discharge, and have high long-term reliability. Such capacitors are used in many fields.
The electric double layer capacitor includes polarizable electrodes, as a positive electrode and a negative electrode, mainly including activated carbon. A withstand voltage of the electric double layer capacitor is 1.2 V when an aqueous electrolytic solution is used, and 2.5 to 3.3 V when an organic electrolytic solution is used.
However, the electric double layer capacitor has a smaller energy density than a secondary battery. Since the energy density is in proportion to capacitance and squared voltage, it is necessary to improve at least one of these elements in order to increase the energy density of the electric double layer capacitor.
In order to increase a voltage of a capacitor, it has been proposed that electric potential of a negative electrode is lowered by allowing lithium ions to be previously stored (pre-doped) into carbon material of the negative electrode. Such a capacitor includes a negative electrode in which lithium ions are stored, a positive electrode as a polarizable electrode, and an electrolytic solution with which the positive electrode and the negative electrode are impregnated and which contains lithium salt. This capacitor is charged and discharged in a range in which lithium ions pre-doped into the negative electrode are not released.
FIG. 2A is a top sectional view of a capacitor in which a lithium ion is used as a cation, which is shown as an example of a conventional capacitor. FIG. 2B is a partially cut-away front view of electrode-wound unit 100 of the capacitor.
As shown in FIG. 2A, the capacitor includes electrode-wound unit 100. Electrode-wound unit 100 is formed such that positive electrode 101 and negative electrode 102 are laminated alternately with separator 103 interposed therebetween to form a laminated body, and the laminated body is wound concentrically. Lithium metals (lithium electrodes) 104 and 105 are disposed, as lithium ion supply sources, at an outer peripheral portion and a central portion of electrode-wound unit 100, respectively. Lithium metal 105 formed at the central portion is supported by tube pole 109. Tube pole 109 simultaneously functions as a shaft pole for supporting electrode-wound unit 100. These are housed in exterior container 106 made of aluminum or iron, the inside of exterior container 106 is filled with an electrolytic solution, and thus the capacitor is configured.
Positive electrode 101 and negative electrode 102 each includes a current collector (not shown) made of porous material provided with through-holes penetrating from a front surface to a back surface thereof. Since the current collector is made of porous material, even if lithium metals 104 and 105 are disposed at the outer peripheral portion and the central portion of electrode-wound unit 100, lithium ions can freely move between the electrodes from lithium metals 104 and 105 through the through-holes of the current collector of electrode-wound unit 100. As a result, lithium ions are previously doped (pre-doped) over entire negative electrode 102.
As shown in FIG. 2B, electrode terminals 107 and 108 are coupled to the current collector of positive electrode 101 and the current collector of negative electrode 102, respectively. Electrode terminals 107 and 108 are led out in the opposite directions along the winding axis direction of cylindrical electrode-wound unit 100. The outermost periphery of electrode-wound unit 100 is fixed by tape 110 so that a wound-shape is held.
In this way, in a conventional capacitor, lithium-ion supply sources are disposed at two portions, that is, at the outer peripheral portion and the central portion of electrode-wound unit 100. This arrangement permits more rapid doping of lithium ions into negative electrode 102 than a method of doping by supplying lithium ions from one lithium-ion supply source. Such a capacitor is disclosed in, for example, Patent Literature 1.
However, in a capacitor having a negative electrode that repeats storing and releasing cations such as lithium ions during charge and discharge, when charge and discharge are repeated after a cell is produced, various properties are deteriorated. As one of the deteriorations of the properties, the electrical potential of the negative electrode, which has been lowered by pre-doping, is raised. When the electric potential of the negative electrode is raised, a potential difference between the positive and negative electrodes is reduced, and the energy density of a storage device is reduced.